This 1Z0-829 practice question tests your understanding of working with arrays and collections. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which statement about TreeSet is true when using a custom Comparator that does not define equals() consistently with compare()?
TreeSet uses equals() to determine duplicates.
Why wrong: Uses compare/compareTo.
TreeSet uses hashCode() for ordering.
Why wrong: Ordering is based on compare/compareTo.
TreeSet considers two elements equal if the comparator returns 0.
Set uses comparator's compare method.
TreeSet throws IllegalArgumentException if equals() is inconsistent.
Why wrong: No exception; but behavior may violate Set contract.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
TreeSet considers two elements equal if the comparator returns 0.
TreeSet uses the compare() method of the provided Comparator (or the natural ordering's compareTo()) to determine element equality. When a custom Comparator returns 0 for two elements, TreeSet treats them as duplicates and does not add the second element, regardless of what equals() returns. This is specified in the Java Collections Framework documentation and is critical for maintaining the Set contract.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
TreeSet uses equals() to determine duplicates.
Why it's wrong here
Uses compare/compareTo.
TreeSet uses hashCode() for ordering.
Why it's wrong here
Ordering is based on compare/compareTo.
TreeSet considers two elements equal if the comparator returns 0.
Why this is correct
Set uses comparator's compare method.
Related concept
Read the scenario before looking for a memorised answer.
TreeSet throws IllegalArgumentException if equals() is inconsistent.
Why it's wrong here
No exception; but behavior may violate Set contract.
Common exam traps
The trap here is that candidates often assume TreeSet uses equals() for duplicate detection because of the general Set contract, but TreeSet (and TreeMap) specifically override that behavior to use compare()/compareTo() instead.
Detailed technical explanation
Under the hood, TreeSet is backed by a TreeMap, which is a Red-Black tree. When adding an element, the tree traverses nodes using the Comparator's compare() method to find the correct insertion point; if compare() returns 0, the element is considered a duplicate and is not inserted. This behavior means that if the Comparator is inconsistent with equals(), the Set may contain elements that are logically equal according to equals() but not according to compare(), or vice versa, which can violate the general contract of the Set interface.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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FAQ
Working with Arrays and Collections — This question tests Working with Arrays and Collections — Read the scenario before looking for a memorised answer..
The correct answer is: TreeSet considers two elements equal if the comparator returns 0. — TreeSet uses the compare() method of the provided Comparator (or the natural ordering's compareTo()) to determine element equality. When a custom Comparator returns 0 for two elements, TreeSet treats them as duplicates and does not add the second element, regardless of what equals() returns. This is specified in the Java Collections Framework documentation and is critical for maintaining the Set contract.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 25, 2026
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